The present invention relates to a light beam scanning apparatus for scanning an object with a light beam to read or record an image or the like, and more particularly to a light beam scanning apparatus which is capable of detecting the scanning position of a light beam with respect to an auxiliary scanning direction for effecting a reading or recording process highly accurately.
There has recently been known a radiation image information recording and reproducing system for producing the radiation-transmitted image of an object using a stimulable phosphor material capable of emitting light upon exposure to stimulating rays. When a certain phosphor is exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, the phosphor stores a part of the energy of the radiation. When the phosphor exposed to the radiation is subsequently exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy of the radiation. The phosphor exhibiting such a property is referred to as a "stimulable phosphor".
In the radiation image recording and reproducing system employing such a stimulable phosphor, the radiation image information of an object such as a human body is stored in a sheet having a layer of stimulable phosphor, and then the stimulable phosphor sheet is scanned with stimulating rays such as a laser beam to cause the stimulable phosphor sheet to emit light representative of the radiation image. The emitted light is then photoelectrically detected to produce an image information signal that is electrically processed for generating image information which is recorded as a visible image on a recording medium such as a photosensitive material or displayed as a visible image on a CRT or the like.
The radiation image recording and reproducing system includes an image reading apparatus for reading image information from a stimulable phosphor sheet on which the image information is recorded. In the image reading apparatus, prior to a reading process (hereinafter referred to as a "main reading mode") in which the sheet is scanned with stimulating light to reproduce a visible image to be observed, a preliminary reading mode is carried out by applying stimulating light, which is of a lower energy level than that of the stimulating light applied in the main reading mode, to the sheet for roughly reading the radiation image information in order to establish reading conditions and/or image processing conditions to be employed in the main reading mode.
More specifically, as shown in FIG. 1 of the accompanying drawings, a laser beam L emitted from a laser oscillating tube 2 is adjusted to a prescribed beam diameter by a beam expander 4. Thereafter, the diameter of the laser beam L is increased by a concave lens 6 which is a preliminary reading optical system inserted in the light path, and then the laser beam L is applied to the reflecting surface of a galvanometer mirror 8. The laser beam L of increased diameter is deflected by the galvanometer mirror 8 as it swings, while the laser beam L is passing through a scanning lens 10 and being reflected by a mirror 12, so as to scan a stimulable phosphor sheet S in a main scanning direction indicated by the arrow A. At the same time, the stimulable phosphor sheet S is fed in an auxiliary scanning direction indicated by the arrow B by a feed means (not shown), so that image information carried on the surface of the sheet S is two-dimensionally scanned by the laser beam L. Light emitted from the stimulable phosphor sheet S upon exposure to the laser beam L is applied through a light guide 14 to a photomultiplier 16, which then photoelectrically converts the light into an electric signal. The electric signal which represents the radiation image information on the sheet S is processed to establish reading conditions and/or image processing conditions to be used in a main reading mode.
After the preliminary reading mode, the stimulable phosphor sheet S is moved back to a reading starting position. Thereafter, the main reading mode is started. The concave lens 6 is retracted out of the light path of the laser beam L into a broken-line position. Then, the laser beam L from the laser oscillating tube 2 is applied, with a prescribed energy level and beam diameter, to the stimulable phosphor sheet S to cause the latter to emit light representative of the recorded radiation image information. The emitted light is then photoelectrically converted into an electric signal based on the reading conditions established in the preliminary reading mode. Thereafter, the electric image signal is processed under the image processing conditions established in the preliminary reading mode for thereby reproducing a visible image of excellent observability.
In the image reading apparatus of this type, if the concave lens 6 is inserted in an inaccurate position in the preliminary reading mode, reading conditions and/or image processing conditions cannot accurately be established. More specifically, if the concave lens 6 is not inserted in a prescribed position in the path of the laser beam L, the laser beam L may be deflected, for example, by the concave lens 6, and may not be applied to a desired position on the stimulable phosphor sheet S. When this happens, the scanning position of the laser beam L in the preliminary reading mode and the scanning position of the laser beam L in the main reading mode do not correspond to each other, making it impossible to establish accurate reading conditions and/or image processing conditions. Another problem is that the laser beam L may be vignetted by the ends of the light guide 14 disposed near the stimulable phosphor sheet S, also making it impossible to establish reading conditions and/or image processing conditions.
If the scanning position of the light beam is deviated from from a proper position not only in the preliminary reading mode but also in a general light beam scanning apparatus, no accurate image can be read or recorded.